Lasers with beam-shape modification

ABSTRACT

A beam control structure for semiconductor lasers that allows modification of the shape of a beam allowing, for example, higher coupling into an optical fiber. The structure may comprise one or more of a tilted patio, a staircase, a reflective roof, and a reflective sidewall.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority to U.S. Provisional PatentApplication No. 61/644,270, filed May 8, 2012, which is herebyincorporated by reference herein in its entirety.

BACKGROUND OF THE DISCLOSURE

The present disclosure relates, in general, to photonic devices, andmore particularly to improved photonic devices and methods forfabricating them.

Semiconductor lasers typically are fabricated on a wafer by growing anappropriate layered semiconductor material on a substrate throughMetalorganic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy(MBE) to form an epitaxial structure having an active layer parallel tothe substrate surface. The wafer is then processed with a variety ofsemiconductor processing tools to produce a laser optical cavityincorporating the active layer and incorporating metallic contactsattached to the semiconductor material. Laser facets are typicallyformed at the ends of the laser cavity by cleaving the semiconductormaterial along its crystalline structure to define edges, or ends, ofthe laser optical cavity so that when a bias voltage is applied acrossthe contacts, the resulting current flow through the active layer causesphotons to be emitted out of the faceted edges of the active layer in adirection perpendicular to the current flow. Since the semiconductormaterial is cleaved to form the laser facets, the locations andorientations of the facets are limited; furthermore, once the wafer hasbeen cleaved, typically it is in small pieces so that conventionallithographical techniques cannot readily be used to further process thelasers.

The foregoing and other difficulties resulting from the use of cleavedfacets led to the development of a process for forming the facets ofsemiconductor lasers through etching. This process, as described in U.S.Pat. No. 4,851,368, also allows lasers to be monolithically integratedwith other photonic devices on the same substrate, the disclosure ofwhich is hereby incorporated herein by reference. This work was furtherextended and a ridge laser process based on etched facets was disclosedin the IEEE Journal of Quantum Electronics, volume 28, No. 5, pages1227-1231, May 1992.

One of the major challenges in the use of semiconductor lasers is themismatch between the output beam from the laser and the medium to whichthe beam is directed or coupled. For example, forming a semiconductorlaser with spot size converters (SSC) can allow more efficient couplingof the laser light to an optical fiber or expand the tolerance foroptical alignment, however, in general there are certain disadvantagesthat come along with forming SSC, such as process complexity anddegradation in laser characteristics. An example of the degradation inlaser characteristics is the increase in the laser threshold current.The following publications discuss the various SSC approaches employed:“Spot-Size Converter Integrated Laser Diodes (SS-LD's)” by Itaya, etal., IEEE Journal of Selected Topics in Quantum Electronics, Volume 3,Number 3, pages 968-974; “A Review on Fabrication Technologies for theMonolithic Integration of Tapers with III-V Semiconductor Devices” byMoerman, et al., IEEE Journal of Selected Topics in Quantum Electronics,Volume 3, Number 6, pages 1308-1320; and “1.3-μm Spot-Size-ConverterIntegrated Laser Diodes Fabricated by Narrow-Stripe Selective MOVPE” byYamazaki, et al., IEEE Journal of Selected Topics in QuantumElectronics, Volume 3, Number 6, pages 1392-1398.

A laser structure formed through a simple process that allows beammodification without significant impact to laser characteristics, suchas laser threshold, is very desirable, and, for example, can lead tovery efficient coupling of the laser beam into an optical fiber with lowcost packaging.

SUMMARY OF THE DISCLOSURE

According to the present disclosure, a semiconductor laser is formedthat allows modification of its output beam.

In one embodiment of the present disclosure, the vertical far-field ofthe laser is modified using an etched facet laser with a patio having atilted angle or a staircase in front of the output facet. In anotherembodiment of the present disclosure, sidewalls are used to modify thehorizontal far-field of the laser in addition to the patio with thetilted angle or staircase. In yet another embodiment, a roof is providedto modify the vertical far-field of the laser. In yet anotherembodiment, a cleaved or etched facet laser is mounted active-side downon a substrate or base such as silicon or aluminum nitride (AlN) withstructures such as a tilted patio or staircase.

For instance, in one embodiment of the present disclosure, asemiconductor chip is disclosed comprising: a substrate; an epitaxiallaser on said substrate; an etched facet; and a structure adjacent saidetched facet, said structure being a patio having one of a downwardlytilt and a downwards staircase having at least one step. Thesemiconductor chip may also comprise reflective sidewalls. Thesemiconductor chip may further comprise a roof in front of said etchedfacet, wherein said roof has a lower reflective surface closer to thehighest point of said etched facet than the lowest point of said etchedfacet. The semiconductor chip may additionally comprise a reflectivecoating deposited on said structure. The semiconductor chip may stillfurther comprise said substrate being selected from the group comprisingInP, GaAs, and GaN.

In another embodiment of the present disclosure, a semiconductor chip isdisclosed comprising: a substrate; an epitaxial laser on said substrate;an etched facet; and a roof in front of said etched facet, wherein saidroof has a lower reflective surface closer to a highest point of saidetched facet than a lowest point of said etched facet. The semiconductorchip may also comprise reflective sidewalls. The semiconductor chip mayfurther comprise said substrate being selected from the group comprisingInP, GaAs, and GaN.

In yet another embodiment of the present disclosure, a semiconductorchip is disclosed comprising: a substrate; an epitaxial laser on saidsubstrate; an etched facet having an angle other than 90° to a plane ofthe substrate; a laser beam impinging on said etched facet below acritical angle of said etched facet; and a reflective structure adjacentsaid etched facet. The semiconductor chip may also comprise saidstructure being a titled patio. The semiconductor chip may furthercomprise reflective sidewalls, wherein said sidewalls may be separatedfrom said etched facet with a gap. The semiconductor chip mayadditionally comprise said structure being a staircase containing atleast one step. The semiconductor chip may still further comprise saidsubstrate being selected from the group comprising InP, GaAs, and GaN.

In yet another embodiment of the present disclosure, a hybrid assemblyis disclosed comprising: a base with a reflective surface of a patiohaving one of a downwardly tilt and a downwards staircase with at leastone step; and a laser with an active layer and at least one facetpositioned active-side down on said base; wherein said at least onefacet is positioned adjacent to said reflective surface. The hybridassembly may also comprise said base being either AlN or Si. The hybridassembly may further comprise said at least one facet being an etchedfacet, further comprising a reflective structure adjacent said etchedfacet. The hybrid assembly may additionally comprise said laser beingformed from a laser structure epitaxially deposited on a substrateselected from the group comprising InP, GaAs, and GaN. The hybridassembly may still further comprise said base further including astopper.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and additional objects, features and advantages of thepresent disclosure will become apparent to those of skill in the artfrom the following detailed description of the present disclosure takenwith the accompanying drawings, which are briefly described as follows.

FIG. 1 (a) is a cross-section of a semiconductor laser with both frontand back facets formed through cleaving, and FIG. 1 (b) is thecorresponding vertical far-field (VFF) from either the front or backfacets obtained through RSoft Finite-Difference Time-Domain (FDTD)simulations.

FIG. 2 (a) is a cross-section of a semiconductor laser with both frontand back facets formed through etching, with a flat patio of 2 μmadjacent the front facet, and FIG. 2 (b) contains the corresponding VFFto this structure in solid line obtained through RSoft FDTD simulationsand the VFF from FIG. 1 (b) in dashed line for reference.

FIG. 3 (a) is a cross-section of a semiconductor laser with both frontand back facets formed through etching, with a flat patio of 10 μmadjacent the front facet, and FIG. 3 (b) contains the corresponding VFFfor this structure in solid line obtained through RSoft FDTD simulationsand the VFF from FIG. 1 (b) in dashed line for reference.

FIG. 4 (a) is a cross-section of a semiconductor laser with both frontand back facets formed through etching, with a 10° tilted patio oflength 10 μm adjacent the front facet, and FIG. 4 (b) contains thecorresponding VFF for this structure in solid line obtained throughRSoft FDTD simulations and the VFF from FIG. 1 (b) in dashed line forreference.

FIG. 5 (a) is a cross-section of a semiconductor laser with both frontand back facets formed through etching, with a staircase adjacent thefront facet, where each step in the staircase is 0.6 μm in height and2.5 μm in width, and FIG. 5 (b) contains the corresponding VFF for thisstructure in solid line obtained through RSoft FDTD simulations and theVFF from FIG. 1 (b) in dashed line for reference.

FIG. 6 (a) is a cross-section of a semiconductor laser with both frontand back facets formed through etching, with a staircase adjacent thefront facet, where each step in the staircase is 0.6 μm in height and2.5 μm in width, and a 1 μm thick “roof” located above the staircasereflective to the laser light on the side of the roof facing thestaircase, having a length of 3.75 μm, positioned such that incross-section, the lower left corner of the roof is 4.75 μm above theedge of the first step, and FIG. 6 (b) contains the corresponding VFFfor this structure in solid line obtained through RSoft FDTD simulationsand the VFF from FIG. 1 (b) in dashed line for reference.

FIG. 7 shows a perspective view of a ridge laser front facet with astaircase adjacent to the front facet, used to control the beamvertically.

FIG. 8 (a) is a top view of a semiconductor laser with reflective angledsidewalls placed in front of the front facet; FIG. 8 (b) shows RSoftFDTD simulations of the intensity of light for the light exiting thefront facet and being modified by the reflective sidewalls; and FIG. 8(c) shows the horizontal far-field (HFF) modified by the reflectivesidewalls in a solids line while the HFF corresponding to the laserwithout any reflective sidewalls is shown in dashed lines for reference.

FIG. 9 (a) is a top view of a semiconductor laser with reflectiveparallel sidewalls placed in front of the front facet; FIG. 9 (b) showsRSoft FDTD simulations of the intensity of light for the light exitingthe front facet and being modified by the reflective sidewalls; and FIG.9 (c) shows the HFF modified by the reflective sidewalls in a solidsline while the HFF corresponding to the laser without any reflectivesidewalls is shown in dashed lines for reference.

FIG. 10 shows a perspective view of a ridge laser front facet with astaircase of a foot and three steps adjacent to the front facet, thefirst and second step being flat, while the third being flat and thentilted towards the substrate, used to control the beam vertically as inFIG. 5 (a), but also including sidewalls similar to that in FIG. 8 thatare used for control the beam in the horizontally.

FIG. 11 shows a perspective view of a ridge laser front facet with astaircase and a roof, used to control the beam vertically as in FIG. 6(a), but also including sidewalls similar to that in FIG. 8 that areused for control the beam in the horizontally.

FIG. 12 (a) is a cross-section of a semiconductor laser with a frontetched facet at an angle A from a perpendicular line to the plane of thesubstrate and back etched facet at or close to perpendicular to thesubstrate, with a flat patio of 10 μm adjacent the front facet, and FIG.12 (b) contains the corresponding VFF for this structure in solid lineobtained through RSoft FDTD simulations and the VFF from FIG. 1 (b) indashed line for reference.

FIG. 13 (a) is a cross-section of a semiconductor laser with a frontetched facet at an angle B from a perpendicular line to the plane of thesubstrate and back etched facet at or close to perpendicular to thesubstrate, with a flat patio of 10 μm adjacent the front facet, and FIG.13 (b) contains the corresponding VFF for this structure in solid lineobtained through RSoft FDTD simulations and the VFF from FIG. 1 (b) indashed line for reference.

FIG. 14 (a) is a cross-section of a semiconductor laser with a frontetched facet at an angle A from a perpendicular line to the plane of thesubstrate and back etched facet at or close to perpendicular to thesubstrate, with a staircase adjacent the front facet, where each step inthe staircase is 0.6 μm in height and 2.5 μm in width, and FIG. 14 (b)contains the corresponding VFF for this structure in solid line obtainedthrough RSoft FDTD simulations and the VFF from FIG. 1 (b) in dashedline for reference.

FIG. 15 shows a cross-sectional view of a cleaved facet laser mountedactive-side down on a silicon base, the silicon base containing astaircase, and the laser is positioned so that the facet is adjacent tothe silicon staircase.

FIG. 16 (a) is a cross-section of a semiconductor laser with both frontand back facets formed through etching, with a staircase adjacent thefront facet, a silicon base containing a staircase, and the laser ismounted active-side down on the silicon base and positioned so that thefront facet is also adjacent to the silicon staircase, and FIG. 16 (b)contains the corresponding VFF for this structure in solid line obtainedthrough RSoft FDTD simulations and the VFF from FIG. 1 (b) in dashedline for reference.

FIG. 17 (a) is a top view of a semiconductor laser with reflectivecurved sidewalls placed in front of the front facet; FIG. 17 (b) showsRSoft FDTD simulations of the intensity of light for the light exitingthe front facet and being modified by the reflective sidewalls; and FIG.17 (c) shows the HFF modified by the reflective sidewalls in a solidsline while the HFF corresponding to the laser without any reflectivesidewalls is shown in dashed lines for reference.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 (a) shows a semiconductor laser 100 formed through cleaving ofthe front 130 and back 110 facets. The laser structure comprises asubstrate 120 with epitaxially deposited layers that allow the formationof a lower cladding layer 140, which may extend into the substrate or becompletely epitaxially deposited as shown in FIG. 1 (a) and of thickness1.83 μm, an active region 180 of 0.34 μm thickness, and an uppercladding layer 160 of 1.83 μm. The laser emits laser light at around1310 nm. FIG. 1 (b) shows the vertical far-field (VFF) obtained throughRSoft Finite-Difference Time-Domain (FDTD) simulations from either thefront or back facet of the structure in FIG. 1 (a).

FIG. 2 (a) shows a cross-section of a semiconductor laser 200 formedthrough etching of the front 230 and back facets 210 at or close toperpendicular to the plane of the substrate 120, which is usually adeviation of the etched facet of up to 3° from the normal to the planeof the substrate. Examples of processes of forming etched facet lasersare described in U.S. patent application Ser. No. 11/356,203 or U.S.Pat. No. 8,130,806, both of which are assigned to the assignee of thepresent application and the disclosures of which is hereby incorporatedby reference in its entirety. The etched facets are typically formed byetching through the upper cladding layer, the active region, and atleast part of the lower cladding. The laser chip is singulated at 270 sothat the patio 250 adjacent the front facet 230 is 2 μm wide (2 μm isthe horizontal distance between the front facet and the singulationplane 270). FIG. 2 (b) shows the VFF obtained through RSoft FDTDsimulations from the front facet in a solid line and the VFF from FIG. 1(b) in dashed lines for reference. There is only a small differencebetween the two VFF curves.

FIG. 3 (a) shows a cross-section of a semiconductor laser 300 formedthrough etching of the front 230 and back 210 facets. The laser chip issingulated at 370 so that the patio 350 adjacent the front facet 230 is10 μm wide. FIG. 3 (b) shows the VFF obtained through RSoft FDTDsimulations from the front facet in a solid line and the VFF from FIG. 1(b) in dashed lines for reference. There is considerable differencebetween the two VFF curves. The VFF in the solid line shows asignificant narrowing of its main lobe 380 full-width half-maximum(FWHM) compared to the dashed line. Furthermore, the VFF in the solidline shows a significant degree of beam pointing that is around 10° fromthe center and the presence of a side lobe 382. A narrow FWHM is veryuseful in, for example, allowing high coupling efficiency to an opticalfiber. However, the beam pointing causes difficulty and incompatibilitywith most traditional approaches of packaging lasers and coupling to anoptical fiber.

FIG. 4 (a) shows a cross-section of a semiconductor laser 400 formedthrough etching of the front 230 and back 210 facets. The laser chip issingulated at 470 so that the patio 450 adjacent to the front facet 230is 10 μm wide, but the patio 450 is also titled at 10° downwards towardsthe substrate 120. FIG. 4 (b) shows the VFF obtained through RSoft FDTDsimulations from the front facet in a solid line and the VFF from FIG. 1(b) in dashed lines for reference. The VFF in the solid line shows asignificant narrowing of the FWHM of its main lobe 480, compared to thedashed line. However, unlike in FIG. 3 (b), the main lobe 480 of the VFFin the solid line is centered and does not show any significant beampointing. The narrow FWHM of the main lobe in the VFF and lack ofbeam-pointing is very useful in, for example, allowing high couplingefficiency to an optical fiber with traditional packaging of thesemiconductor laser chip for coupling to an optical fiber. In general,the power in the side lobe 482 will not couple into an optical fiber,for example, as efficiently as the main lobe, and, as such, it isdesirable to minimize the side lobe and maximize the main lobe forhighest efficiency coupling, for example, to an optical fiber.

FIG. 5 (a) shows a cross-section of a semiconductor laser 500 formedthrough etching of the front 230 and back 210 facets. The laser chip issingulated at 570 so that the staircase adjacent to the front facet is10 μm wide. The staircase has a foot 505 and three flat steps, 510, 520,and 530, and the staircase goes downwards towards the substrate. Thesurface of step 510 is lower than the active region and at least part ofthe lower cladding layer at the etched front facet 230, the surface ofstep 520 is lower than 510, and the surface of step 530 is lower than520. Each step is 2.5 μm in width and 0.6 μm in height. The foot 505 canbe lower than the surface of step 510, however, it can only be slightlyhigher than step 510, so long as it does not interfere with the beam inany significant way. FIG. 5 (b) shows the VFF obtained through RSoftFDTD simulations from the front facet in a solid line and the VFF fromFIG. 1 (b) in dashed lines for reference. The VFF in the solid lineshows a significant narrowing of the FWHM of its main lobe 580 comparedto the dashed line. As in FIG. 5 (b), the main lobe 580 of the VFF inthe solid line is centered and does not show any significant beampointing. Even a single step in the staircase has shown significantimpact on eliminating the beam pointing. The side lobe 582 is reduced inintensity compared to 482.

FIG. 6 (a) shows a cross-section of a semiconductor laser 600 formedthrough etching of the front 230 and back 210 facets. The laser chip issingulated at 670 so that the staircase adjacent to the front facet is10 μm wide. The staircase has a foot 605 and three flat steps, 610, 620,and 630, and the staircase goes downwards towards the substrate. Thesurface of step 610 is lower than the active region and at least part ofthe lower cladding layer at the etched front facet 230, the surface ofstep 620 is lower than 610, and the surface of step 630 is lower than620. Each step is 2.5 μm in width and 0.6 μm in height. In addition, a 1μm thick roof 640 is located above the staircase that is reflective tothe laser light on the side of the roof facing the staircase, having alength of 3.75 μm, positioned such that in cross-section, the lower leftcorner of the roof is 4.75 μm above the edge of the first step in thestaircase. FIG. 6 (b) shows the VFF obtained through RSoft FDTDsimulations from the front facet in a solid line and the VFF from FIG. 1(b) in dashed lines for reference. The VFF in the solid line shows asignificant narrowing of the FWHM of its main lobe 680 compared to thedashed line. As in FIGS. 4 (b) and 5 (b), the main lobe 680 of the VFFin the solid line is centered and does not show any significant beampointing. However, more of the power is concentrated in the main lobe680 and the side lobe 682 is further reduced compared to 482 and 582,although another small side lobe 684 is present.

FIG. 7 shows a perspective view of a ridge laser 700 with the frontfacet 230 adjacent to a two-step staircase and a foot. The foot 705 isdepicted as a first surface with its plane being the lowest point of thefront etched facet 230. The first step 710 and second step 720 both haveflat surfaces. A tilted surface 730 is present that is angled downwardstowards the substrate. The chip is singulated at 740. Although a ridge790 laser is depicted, it will be understood that other types of lasersmay be fabricated utilizing the features described herein. For example,the laser structure can also be a buried heterostructure (BH) laser. Thetype of laser can, for example, be a Fabry Perot (FP) laser or adistributed feedback (DFB) laser. The foot 705 can have a surface lowerthan the surface of step 710 however, it can only be slightly higherthan step 710 so long as it does not interfere with the beam in anysignificant way. If the foot is fabricated as depicted in FIG. 7 with asmooth reflective surface, it may be used as the first step in thestaircase.

In an experiment conducted, two types of ridge lasers were fabricated.Type 1 was of the kind shown in FIG. 2 (a), and Type 2 was as shown inFIG. 5 (a) with three steps in the staircase. As is conventional in thefabrication of solid state ridge lasers, the substrate may be formed,for example, of a type III-V compound, or an alloy thereof, which may besuitably doped. The substrate includes a top surface on which isdeposited, as by an epitaxial deposition such as Metalorganic ChemicalVapor Deposition (MOCVD) or Molecular Beam Epitaxy (MBE), a successionof layers. The laser structure was a 1310 nm emitting epitaxialstructure with the following layers on an InP substrate: n-InP lowercladding; AlGaInAs lower graded region; an active region containingcompressively strained AlGaInAs quantum wells, each sandwiched bytensile strained AlGaInAs barriers; AlGaInAs upper graded region; p-InPupper cladding; and highly p-doped InGaAs cap layer. Wafer-level testingdetermined that the ridge laser electronic characteristics, such asthreshold current, were very similar for Type 1 and 2 lasers. Type 1 and2 lasers were packaged in a TO-56 can with a 1.5 mm lens of refractiveindex 1.496 without AR coating. The packaged lasers were coupled to anoptical fiber at an optimal position and the slope efficiency (SE) inthe fiber (the amount of coupled laser power in the fiber divided by theamount current above threshold applied to the laser diode) determined.The average SE in the fiber for Type 1 was 0.0737 W/A, while the averageSE in the fiber for Type 2 was 0.0970 W/A, an increase in couplingefficiency due to the staircase adjacent the front facet of more than31%.

FIG. 8 (a) shows a top view of a semiconductor laser 800 with a 2 μmwide ridge 790. This laser has a horizontal far-field (HFF) shown inFIG. 8 (c) with dashed lines obtained through RSoft FDTD simulations,when there are no features in front of the front-facet, as would be thecase for FIG. 1 (a) or 2 (a). Reflective sidewalls 810 and 820 areplaced in front of the front-etched-facet 230 at a gap 840 of 2 μm toallow electrical isolation between the laser and the reflectivesidewalls. The sidewall structure has a length 860 of 13 μm. Thereflective surface 810 is at an angle 830 of 75° to the plane of thefront-etched-facet. The reflective surface 820 is at an angle 835 of 75°to the plane of the front-etched-facet. The gap 850 between the tworeflective sidewalls is 6 μm at their closest point to the front etchedfacet. FIG. 8 (b) shows RSoft FDTD simulations of the ridge laser andthe reflective sidewalls. FIG. 8 (c) shows the impact of the reflectivesidewalls on the HFF in solid line and that the HFF has significantlynarrowed over that of the laser without the reflective sidewalls. Anarrower HFF from a semiconductor laser has many applications, includingbetter coupling into an optical fiber.

FIG. 9 (a) shows a top view of a semiconductor laser 900 with a 2 μmwide ridge 790. This laser has a horizontal far-field (HFF) shown inFIG. 9 (c) with dashed lines obtained through RSoft FDTD simulations,when there are no features in front of the front-facet, as would be thecase for FIG. 1 (a) or 2 (a). Reflective sidewalls 910 and 920 areplaced in front of the front etched facet at a gap 940 of 2 μm to allowelectrical isolation between the laser and the reflective sidewalls. Thesidewall structure has a length 960 of 13 μm and the reflective surface910 is at an angle 930 of 90° to the plane of the front etched facet.The reflective surface 920 is parallel to 910. The gap 950 between thetwo reflective sidewalls is 6 μm. FIG. 9 (b) shows RSoft FDTDsimulations of the ridge laser and the reflective sidewalls. FIG. 9 (c)shows the impact of the reflective sidewalls on the HFF in solid lineand that two district lobes have been formed as a result of thesidewalls. Splitting a laser beam into two or more lobes has manyapplications, such as providing light to two or more waveguides,respectively.

FIG. 10 shows a perspective view of a ridge laser 1000 front facet 230with a staircase of two steps and a foot adjacent to the front facet230. The first flat surface is a foot 1005 defined by the lowest pointof the front etched facet 230. The first step 1010 and second step 1020have flat surfaces. A tilted surface 1030 angled downwards towards thesubstrate and the chip is singulated at 1040. The structure includesreflective sidewalls 810 and 820. The staircase structure allow more ofthe laser light power from the front etched facet to remain along a lineextending from the ridge and parallel to the ridge 790, and as such theimpact of the reflective sidewalls 810 and 820 is more pronounced thancould be the case, for example, of the structure in FIG. 3 (a). The foot1005 can have a surface lower than the surface of step 1010, however, itcan only be slightly higher than step 1010, so long as it does notinterfere with the beam in any significant way. If the foot is designedas depicted in FIG. 10 as having a smooth reflective surface, it may beused as the first step in the staircase.

In order to form the reflective surfaces for the sidewalls 810 and 820,sputtering of a reflective metal is used with a lift-off process.Alternatively, evaporated metal is used with a lift-off process, but thesubstrate is rocked during evaporation to allow good coverage on thesidewalls 810 and 820, as well as the flat surface 1010 and 1020. Itwill be understood that other types of reflective films can be depositedon the sidewalls.

FIG. 11 shows a perspective view of a laser 1100, identical to FIG. 10,but including a roof 1110 to further concentrate laser light power inthe main lobe and reduce the side lobe, as discussed above whendescribing FIGS. 6 (a) and (b). The roof is deposited from a reflectivematerial, such as a metal, and using a process similar to that used informing metal bridges in semiconductors (see, for example,http://www.microchem.com/Appl-IIIVs-Airbridges.htm).

FIG. 12 (a) shows a cross-section of a semiconductor laser 1200 formedthrough etching of the front facet 1230 at an angle A of 10° to thenormal to the plane of the substrate and etching of the back facet 210at or close to perpendicular to the plane of the substrate. The laserchip is singulated at 370 so that the patio 350 adjacent the front facet1230 is 10 μm wide. FIG. 12 (b) shows the VFF obtained through RSoftFDTD simulations from the front facet in a solid line and the VFF fromFIG. 1 (b) in dashed lines for reference. There is considerabledifference between the two VFF curves. The VFF in the solid line shows asignificant narrowing of the FWHM of its main lobe 1280 compared to thedashed line. Furthermore, the VFF in the solid line shows a significantdegree of beam pointing that is around 10° from the center and thepresence of a side lobe 1282. The side lobe 1282 is larger than sidelobe 382.

FIG. 13 (a) shows a cross-section of a semiconductor laser 1300 formedthrough etching of the front facet 1330 at an angle B of 10° to thenormal to the plane of the substrate and etching of the back facet 210at or close to perpendicular to the plane of the substrate. The laserchip is singulated at 370 so that the patio 350 adjacent the front facet1330 is 10 μm wide. FIG. 13 (b) shows the VFF obtained through RSoftFDTD simulations from the front facet in a solid line and the VFF fromFIG. 1 (b) in dashed lines for reference. There is considerabledifference between the two VFF curves. The VFF in the solid line shows asignificant narrowing of the FWHM of its main lobe 1380 compared to thedashed line. Furthermore, the VFF in the solid line shows a significantdegree of beam pointing that is around 10° from the center and thepresence of a side lobe 1382. The side lobe 1382 is larger than sidelobe 382.

FIG. 14 (a) shows a cross-section of a semiconductor laser 1400 formedthrough etching of the front 1230 at an angle A of 10° to the normal tothe plane of the substrate and etching of the back 210 facet at or closeto perpendicular to the plane of the substrate. The laser chip issingulated at 570 so that the staircase adjacent to the front facet is10 μm wide. The staircase has a foot 505 and three flat steps, 510, 520,and 530, and the staircase goes downwards towards the substrate. Step510 is lower than the active region and at least part of the lowercladding layer at the etched front facet 1430, the surface of step 520is lower than 510, and the surface of step 530 is lower than 520. Eachstep is 2.5 μm in width and 0.6 μm in height. The foot 505 can be lowerthan the surface of step 510, however, it can only be slightly higherthan step 510 so long as it does not interfere with the beam in anysignificant way. FIG. 14 (b) shows the VFF obtained through RSoft FDTDsimulations from the front facet in a solid line and the VFF from FIG. 1(b) in dashed lines for reference. The VFF in the solid line shows asignificant narrowing of its main lobe 1480 FWHM compared to the dashedline. As seen in FIG. 14 (b), the main lobe 1480 of the VFF in the solidline is centered and does not show any significant beam pointing. Even asingle step in the staircase has shown significant impact on eliminatingthe beam pointing. The side lobe 1482 is reduced in intensity comparedto 482.

FIG. 15 shows a cross-sectional view of a hybrid assembly 1500 of thecleaved facet laser of FIG. 1 (a) mounted active-side down on a siliconsubstrate or base. Other type of substrates or base materials, such asAlN can be substituted for the silicon base. The silicon base is cut orsingulated at 1575. The silicon base contains a downward staircasestructure. The staircase structure depicted in FIG. 15 shows a foot 1505and three steps 1510, 1520, and 1530, where step 1510 is higher than1520, and step 1520 is higher than step 1530. Each step is 2.5 μm inwidth and 0.6 μm in height. The foot 1505 can be lower than the surfaceof step 1510, however, it can only be slightly higher than step 1510 solong as it does not interfere with the beam in any significant way. Thecleaved facet laser is carefully positioned so that the facet isadjacent to the silicon staircase so that so that the staircase adjacentto the front facet is 10 μm wide. A VFF similar to that of the solidline in FIG. 5 (b) is obtained through RSoft FDTD simulations. Thesilicon base may further incorporate a stopper 1590 that allows thecleaved facet laser to be positioned with high accuracy on the siliconbase.

FIG. 16 (a) shows a cross-sectional view of a hybrid assembly 1600 ofthe etched facet laser of FIG. 5 (a) mounted active-side down on asilicon base. The silicon base is cut or singulated at 1575. The siliconbase contains a downward staircase structure. The staircase structuredepicted in FIG. 16 (a) shows a foot 1505 and three steps 1510, 1520,and 1530, where step 1510 is higher than 1520, and step 1520 is higherthan step 1530. Each step is 2.5 μm in width and 0.6 μm in height. Thefoot 1505 can be lower than the surface of step 1510, however, it canonly be slightly higher than step 1510 so long as it does not interferewith the beam in any significant way. The laser chip is singulated at570 so that the staircase adjacent to the front facet is 10 μm wide. Thestaircase has a foot 505 and three flat steps, 510, 520, and 530, andthe staircase goes downwards towards the substrate 120. The surface ofstep 510 is lower than the active region and at least part of the lowercladding layer at the etched front facet 230, the surface of step 520 islower (towards substrate 120) than 510, and the surface of step 530 islower (towards substrate 120) than 520. Each step is 2.5 μm in width and0.6 μm in height. The foot 505 can be lower than the surface of step 510however, it can only be slightly higher than step 510, but so long as itdoes not interfere with the beam in any significant way. The etchedfacet laser is carefully positioned so that the facet is adjacent to thesilicon staircase so that so that the silicon staircase adjacent to thefront facet is about 10 μm wide. FIG. 16 (b) shows the VFF obtainedthrough RSoft FDTD simulations from the front facet in a solid line andthe VFF from FIG. 1 (b) in dashed lines for reference. The VFF in thesolid line shows a significant narrowing of its main lobe 1680 FWHMcompared to the dashed line. As seen in FIG. 16 (b), the main lobe 1680of the VFF in the solid line is centered and does not show anysignificant beam pointing. The side lobe 1682 is reduced in intensitycompared to 482. Another small side lobe 1684 is present.

FIG. 17 (a) shows a top view of a semiconductor laser 1700 with a 2 μmwide ridge 790. This laser has a horizontal far-field (HFF) shown inFIG. 17 (c) with dashed lines obtained through RSoft FDTD simulations,when there are no features in front of the front-facet, as would be thecase for FIG. 1 (a) or 2 (a). Reflective curved sidewalls 1710 and 1720are placed in front of the front etched facet at a gap 1740 of 6 μm toallow electrical isolation between the laser and the reflectivesidewalls. The curved sidewall structure has a length 1760 of 4 μm and aradius of curvature of 3 μm with gap 1750 of 7 μm. FIG. 17 (b) showsRSoft FDTD simulations of the ridge laser and the reflective sidewalls.FIG. 17 (c) shows the impact of the reflective sidewalls on the HFF insolid line and that the HFF has narrowed over that of the laser withoutthe reflective sidewalls. This illustrates that many shapes of sidewallsare possible beyond simple straight ones.

Devices with angled etched front facets, such as those depicted in FIGS.12 (a), 13 (a), and 14 (a), have their facet angled such that a laserbeam impinges on the front facet at an angle below the critical angle ofthe front facet. This allows at least partial transmission from thesefacets. DFB lasers with antireflection coated angled front facetsperform particularly well with this approach.

Prior art devices using semiconductor laser with spot size converters(SSC) allowed the emitted beam from the laser to have a shape that wasmodified. However, the incorporation of the SSC came at the loss oflaser performance. For example, a laser with a SSC would have athreshold current that was higher than the same laser without the SSC.One of the beneficial characteristics of the present disclosure is thatthe laser threshold current is not impacted in any significant way whenthe staircase, roof, or sidewalls are added to the laser. Themodification of the beam shape allows several benefits, for example,higher coupling efficiency to an optical fiber or an optical waveguide,or expanded tolerance for optical alignment.

Although, for example, laser 200 has been described as having its backfacet 210 formed through etching, it will be understood that the backfacet could alternatively be formed through cleaving.

When the angle of the surface 730 or 1030 is around 45° or highertowards the substrate, it does not contribute in the control ormodification of the laser beam in any significant way, however, it cansignificantly increase the tolerance in singulation position, that is,the distance between the plane of 740 and that of the plane offront-etched-facet 230 can have a larger tolerance and make thesingulation process easier to carry out.

The different depth levels of each flat surface of a step in thestaircase structure are defined with high accuracy through epitaxialgrowth. In the case of InP based laser, epitaxial material is grown byalternating two materials which have wet-etch selectivity relative toeach other, for example a 0.58 μm layer of InP alternating with a thinlayer of around 0.02 μm of InGaAs or InGaAsP. The two-layers arerepeated to the extent that steps are desired in the staircasestructure. They are typically n-type doped. An n-type lower cladding,undoped active region, p-type upper cladding, and a highly p-typecontact layer are subsequently deposited on top of these layers.

After fabricating the etched facet and the ridge, the staircasestructure in front of the laser is formed by a sequence oflithographical mask definition with photoresist or dielectric followedby a layer-specific wet chemical etching, for example 1:4 HCl—H₃PO₄ forInP etching and 1:1:10 H₂SO₄:H₂O₂:H₂O for InGaAs or InGaAsP etching.

Single longitudinal mode lasers are more desirable thanmulti-longitudinal mode lasers in many applications. One suchapplication is in data communications where longer reaches ofcommunications are obtained with a single longitudinal mode laserscompared to a multi-longitudinal laser. As discussed above, a DFB laserwith one or more of the following: the staircase, roof, and thereflective sidewalls, allow the beam shape from the laser to bemodified. U.S. Pat. No. 7,835,415, assigned to the assignee of thepresent application and the disclosure of which is hereby incorporatedby reference in its entirety, teaches an alternative single longitudinallaser that can be used in conjunction with the present disclosure forlaser beam control.

Semiconductor lasers with high VFF values, such as greater than 40°, canbe designed to have lower threshold currents, which is desirable.However, typically, these lasers have poor coupling to, for example,optical fibers. The present disclosure allows for low threshold currentbenefits of a high VFF, while allowing good coupling efficiency.

Although the present disclosure was described in terms of a 1310 nmemitting InP based laser, it will be understood that other wavelengthson laser structures on InP, as well as other wavelength lasers on othersubstrates, such as violet, blue, and green on laser structures on GaNsubstrates and infrared and red on lasers structures on GaAs substratescan also benefit from the present disclosure.

Although the present disclosure has been illustrated in terms ofpreferred embodiments, it will be understood that variations andmodifications may be made without departing from the true spirit andscope thereof as set out in the following claims.

The invention claimed is:
 1. A semiconductor chip, comprising: asubstrate having a surface; and an epitaxial laser on the surface ofsaid substrate, said epitaxial laser having an etched facet; whereinsaid substrate extends outwardly from said etched facet to form a patiocomprising an exposed reflective surface and a plurality of steps formedtherein, wherein each subsequent step outwardly from said etched facetextends further into said substrate in a direction substantially normalto the surface of said substrate such that light emitted from said laseris reflected off the exposed reflective surface of at least one of theplurality of steps to modify a vertical far field of said laser, andwherein each of said the plurality of steps is clear of obstacles thatmay interfere with said light emitted from said laser.
 2. Thesemiconductor chip of claim 1, further comprising at least onereflective sidewall extending outwardly from said etched facet, eachsidewall having a reflective surface substantially perpendicular to asurface plane of said patio.
 3. The semiconductor chip of claim 2,further comprising a roof disposed opposite from said patio extendingoutwardly from said etched facet, said roof having a reflective surfaceopposing the surface of said patio.
 4. The semiconductor chip of claim1, further comprising a reflective coating deposited on said patio. 5.The semiconductor chip of claim 1, wherein said substrate is selectedfrom the group comprising InP, GaAs, and GaN.
 6. A semiconductor chip,comprising: a substrate; an epitaxial laser on said substrate, saidepitaxial laser having an etched facet; a patio extending outwardly fromsaid etched facet, said patio having an exposed reflective surface and aplurality of steps formed therein, each subsequent step outwardly fromsaid etched facet extending further into said substrate; and a roofextending outwardly from said etched facet, said roof having an exposedreflective surface opposing said reflective surface of said patio;wherein light emitted from said laser is reflected off the exposedreflective surfaces of said roof and at least one of the plurality ofsteps to modify a vertical far field of said laser.
 7. The semiconductorchip of claim 6, further comprising at least one reflective sidewallextending outwardly from said etched facet, each sidewall having areflective surface substantially perpendicular to said reflectivesurface of said patio.
 8. The semiconductor chip of claim 6, whereinsaid substrate is selected from the group comprising InP, GaAs, and GaN.9. A semiconductor chip, comprising: a substrate; an epitaxial laser ona surface of said substrate, said epitaxial laser having an etchedfacet, said etched facet having a surface at an angle other than 90° tosaid surface of said substrate such that a laser beam generated by saidepitaxial laser impinges on said etched facet below a critical angle ofsaid etched facet; and a structure adjacent said etched facet, saidstructure having at least two opposing reflective surfaces extendingoutwardly from said etched facet.
 10. The semiconductor chip of claim 9,wherein said structure further comprises a tilted patio having areflective surface.
 11. The semiconductor chip of claim 10, wherein saidat least two opposing reflective surfaces comprises at least tworeflective sidewalls, each sidewall having a reflective surfacesubstantially perpendicular to said reflective surface of said patio.12. The semiconductor chip of claim 11, wherein each sidewall isseparated from said etched facet with a gap.
 13. The semiconductor chipof claim 9, wherein said structure further comprises a staircasecontaining a plurality of steps having reflective surfaces, eachsubsequent step outwardly from said etched facet extending further intosaid substrate.
 14. The semiconductor chip of claim 13, wherein said atleast two opposing reflective surfaces comprises at least two reflectivesidewalls, each sidewall having a reflective surface substantiallyperpendicular to said reflective surfaces of said steps.
 15. Thesemiconductor chip of claim 14, wherein each sidewall is separated fromsaid etched facet with a gap.
 16. The semiconductor chip of claim 9,wherein said substrate is selected from the group comprising InP, GaAs,and GaN.
 17. A hybrid assembly, comprising: a base with an exposedreflective surface of a patio having a downwards staircase with aplurality of steps, each subsequent step extending further into saidbase; and a laser with an active layer and at least one etched facet,wherein said laser is mounted on said base; wherein said at least onefacet is positioned adjacent to said exposed reflective surface suchthat light emitted from said laser is reflected off the exposedreflective surface of at least one of the plurality of steps to modify avertical far field of said laser, and wherein each of said the pluralityof steps is clear of obstacles that may interfere with said lightemitted from said laser.
 18. The hybrid assembly of claim 17, whereinsaid base is either AlN or Si.
 19. The hybrid assembly of claim 17,further comprising a reflective structure adjacent said etched facet.20. The hybrid assembly of claim 17, wherein said laser is formed from alaser structure epitaxially deposited on a substrate selected from thegroup comprising InP, GaAs, and GaN.
 21. The hybrid assembly of claim17, wherein said base further includes a stopper against which saidlaser is positioned on said base.